programming with max/msp - Virtual Sound

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Chapter 4T - Control signals 4.1 CONTROL SIGNALS: STEREO PANNING As we have seen throughout this book, envelope generators can be used to vary the parameters of a sound, such as frequency or amplitude, in time. Signals that come from envelope generators – signals that are expressly produced to vary parameter values rather than to produce sound – are called control signals. To this point, we have used line segments and exponential curves to describe control signals. These techniques are efficient because they require only a few values to describe parameter changes; think of the four segments that we use for ADSR envelopes, or of the numbers that define an exponential curve, which can completely describe a glissando. Other control signals, however, may need to vary in more complex ways. Take the vibrato associated with a string instrument as an example: there is a continuous change in the frequency of a note played with vibrato that can best be described as an oscillation around a central pitch. To simulate such a vibration using an envelope, we might need to use tens, or even hundreds, of line segments, which would be both tedious and impractical. Instead of resorting to such primitive methods, we might instead choose to use a control oscillator, an oscillator whose output is produced for the sole purpose of providing parameter values to audio oscillators or other parameterized devices used in sound synthesis and signal processing. Control oscillators are usually Low Frequency Oscillators (LFOs); their frequency is usually below 30 Hz. They produce continuously changing control values that trace waveforms in the same way that audio oscillators do. Every instantaneous amplitude of a wave generated by a control oscillator corresponds to a numeric value that can be applied to audio parameters as needed. Here is an example demonstrating the use of an LFO: in Figure 4.1, you see a graphic representation of an LFO controlling the position of a sound in space. This LFO generates a sine wave that oscillates between MIN (representing its minimum value) and MAX (its maximum value). �� Fig. 4.1 An LFO for controlling the position of a sound in space The minimum and maximum values, or the depth of the oscillator, define the limits of the amplitude values being produced, while the rate of the oscillator is a value that defines its frequency. from “Electronic Music and Sound Design” Vol. 1 by Alessandro Cipriani and Maurizio Giri © ConTempoNet 2010 - All rights reserved 4T 473
4.2 474 Theory In the example, the instantaneous amplitude values of the sine wave generated by the LFO is used as input to two multipliers that inversely scale the amplitude of an audio oscillator on two output channels. Whenever the control signal reaches MIN (its minimum value), the sound is panned completely left, and whenever it reaches MAX (its maximum value), the sound is panned completely right. While intermediate values are being produced (represented in the figure by the central value INT), the sound is smoothly mixed between the two channels. It should be obvious that it would be possible to use other waveforms (triangle, random, etc.) to control parameter values in the same way; a square wave, for example, could control the location of a sound bouncing between left and right channels without intermediate positions. In this case, there would be no continuous change, as there is when using a sine wave; the values would simply alternate between MIN and MAX. 8 INTERACTIVE EXAMPLE 4A • Panning using different LFO waveforms The rate with which values change depends on the frequency assigned to a given control oscillator. If you use a frequency of 1 Hz, you will move from MAX to MIN and back again in one second; if you use a frequency of .2 Hz, you will have 1 complete oscillation in 5 seconds. What if you use a frequency of 220? In this case, the 220 oscillations per second would be too fast to allow us to hear the location moving between left and right; this frequency would instead enter the audio range and would generate new components in the spectrum of the resulting sound. We will cover this phenomenon, amplitude modulation, in Chapter 10. 8 INTERACTIVE EXAMPLE 4B • Panning using a sine wave LFO at various frequencies By using control oscillators, we can control the depth and the rate of a vibrato, of a tremolo, or of variations in filter parameters, all of which we will cover in the following sections. (...) from “Electronic Music and Sound Design” Vol. 1 by Alessandro Cipriani and Maurizio Giri © ConTempoNet 2010 - All rights reserved Paragraph 4.2 - DC Offset
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CONTENTS Foreword by David Zicarell
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Electronic Music and Sound Design -
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LEARNING AGENDA 1T PREREQUISITES FO
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